Currently, to meet regulatory standards particulate filters are used in conjunction with exhaust systems for engines and particularly exhaust systems for diesel engines to remove contaminants from the exhaust stream. Requirements have been placed on the particulate filters to meet multiple contradictory exacting requirements. For example, the filter is expected to have a sufficient porosity (e.g., generally greater than 55 percent porosity) while still retaining most of the emitted micrometer sized diesel particulates (e.g., generally greater than 90 percent capture of the emitted particulates). The filter is expected to be permeable enough so that excessive back pressure does not occur too quickly, and it is expected that the particulate filter may be loaded with a great amount of soot before being regenerated. The filter is expected to withstand the corrosive exhaust environment for long periods of time. The filter is expected to withstand thermal cycling (i.e., withstand cracking, melting, deactivation, reduced filtration efficiency, or a combination thereof) from the burning off of the soot entrapped in the filter (i.e., regeneration) over thousands of cycles where local temperatures may reach as high as 1400° C. From these stringent criteria, ceramic filters have been the choice of material to develop a diesel particulate filter.
These contradictory requirements force ceramists to look for even more improved materials that retain strength properties while maintaining a high porosity and filtering ability without unduly increasing back pressure. Acicular mullite meets many of these needs; however, there are still areas that may be improved such as resistance to thermal shock, high overall strength, increased porosity, resistance to backpressure build up, or a combination thereof.
U.S. Pat. Nos. 7,485,594 and 7,425,297 disclose various calcination and mullitization temperatures and durations used for making acicular mullite all of which are incorporated by reference herein. Pyzik et al., “Formation mechanism and microstructure development in acicular mullite ceramics fabricated by controlled decomposition of fluorotopaz, available at www.science direct.com, or Journal of the European Ceramic Society 28 (2008) 383-391, May 3, 2007, discloses a method of forming acicular mullite ceramics, incorporated by reference herein. Accordingly, it would be desirable to provide a ceramic material that has a high strength and high resistance to thermal shock, while maintaining a high porosity so that back pressure does not quickly increase.